1. Field of the Invention
The present invention relates to a wireless IC device and, in particular, to a wireless IC device used in an RFID (radio frequency identification) system.
2. Description of the Related Art
As article management systems, there have been developed RFID systems in which a reader/writer, which generates an induction electromagnetic field, and an wireless tag (also called a wireless IC device), which is attached to an article and stores predetermined information, communicate with each other using a non-contact method so as to transmit information. As a wireless tag used in this type of RFID system, Japanese Unexamined Patent Application Publication No. 2001-257292 describes a semiconductor apparatus that has an antenna coil formed on one main surface of an IC chip and is used in a contactless IC card. This semiconductor apparatus wirelessly communicates with a reader/writer using the antenna coil electrically connected to a circuit inside the IC chip.
However, as for the semiconductor apparatus described in Japanese Unexamined Patent Application Publication No. 2001-257292, the size of the antenna coil is small, since the size is the same as that of the IC chip at most. Therefore, the semiconductor apparatus has a problem in that it cannot communicate with a reader/writer if there is a long distance between the semiconductor apparatus and the reader/writer or if the position of the semiconductor apparatus relative to the reader/writer is shifted even a bit. Also, if a signal having a predetermined frequency is transmitted from the antenna coil, it is necessary to set the length of the antenna coil to ½ of a wavelength corresponding to the frequency of the signal. However, the IC chip is small. Therefore, if an attempt is made to dispose an antenna coil having a predetermined length on the IC chip, the widths of electrodes of the coil or the interval between the electrodes must be narrowed. For this reason, a facility for manufacturing antenna coils with high accuracy is required and there remains a possibility that even minute variations of the IC chips cause variations in communication frequency and thus a communication failure occurs.